Reviews — Wachsmiith and Springer — On Crinoids. 221 



ambulacral plates naturally came into the tegmen. The plates of 

 the tegmen were at first small and yielding, as in the Ichthyocrinidee 

 and in most recent Crinoids ; in this state when the arms are open 

 the ventral surface is depressed, when they are closed it bulges 

 upwards. To afford better protection to the viscera the tegminal 

 plates became more solid ; the tegmen being thus less flexible was 

 fixed perforce in its protruded state. The covering -plates of the 

 ambulacra had perhaps been closed from the beginning, but as, 

 through the upswelling of the tegmen, they were now moi'e exposed, 

 further protection was needed. Consequently they were lowered 

 beneath the surface and, starting from the solid orals, interambu- 

 lacral plates closed in over them. Certain of the covering-plates 

 however, especially, it would appear, the axillary pieces, which 

 perhaps could not so easily be covered by other plates, became 

 much stouter and were still exposed on the surface as solid radial 

 dome plates. In any form highly developed along these lines, e.g. 

 Batocrinus, the food-grooves, water- vessels and blood-vessels are 

 sunk right beneath the tegmen and are enclosed in a tube consisting 

 of alternating ambulacral or covering plates above and adambulacral 

 or side jDlates below. The interambulacral plates of the tegmen 

 send curious extensions into the interior of the calyx, and these 

 extensions, spreading out, form what was formerly supposed to be a 

 disk. We may, with Messrs. Wachsmuth and Springer, regard the 

 extensions as caused by the perforation of the plates for water- 

 canals ; or we may regard them as simple processes for the purpose of 

 adding strength, without forgoing lightness, by a system of girders. 

 In the Inadunata Fistulata the dorsal cup never extended beyond 

 the radials, and the tegmen was not developed to the same extent as 

 in the Camerata. The orals did not, however, always persist in the 

 simple stage in which they occur in the Larviformia ; in many cases 

 they were, in the opinion of the authors, entirely resorbed, while 

 their places were taken "by large covering plates, of which the 

 proximal ones joined in the center." Ambulacral and sometimes a 

 small number of interambulacral plates occur in the tegmen of the 

 Cyathocrinidge ; besides there are four large plates, one in each 

 interradius except the posterior, which rest against the radials, meet 

 laterally beneath the ambulacra, and may be covered to a varying 

 extent by small interambulacral plates. In the posterior interradius 

 there lies, between the ventral sac and the mouth, a plate often very 

 similar in shape and position to tliese four. This plate is stated by 

 our authors to be " profusely perforated " in vai-ious Lower Carbon- 

 iferous Cyathocrinid^, and they state that on either side of it lies 

 a small narrow plate which meets the large plate of the adjacent 

 inteiTadius beneath the ambulacrum. These two narrow plates are 

 quite new discoveries. Once upon a time the five large plates were 

 considered to be orals, a view subsequently abandoned by Wachsmuth 

 and Springer and P. H. Carpenter owing to the passage of the 

 ambulacra over and not between the edges of the plates. Wachsmuth 

 and Springer have since then regarded them as interradials ; this 

 view they now drop because the plates "support the ambulacra and 



